Ed. Note: Chris Halkides has been kind enough to try to make us lawyers smarter by dumbing down science enough that we have a small chance of understanding how it’s being used to wrongfully convict and, in some cases, execute defendants. Chris graduated from the University of Wisconsin-Madison with a Ph.D. in biochemistry, and teaches biochemistry, organic chemistry, and forensic chemistry at the University of North Carolina, Wilmington.
The efficacy of marijuana testing is a very significant problem. Without an appreciation of how and why drug tests produce false positive, and how to argue against them, they become a “black box” that we simply assume to be correct for lack of the knowledge to challenge. Chris explains what we, as lawyers, generally fail to understand so we can more effectively defend our clients.
For forensic analyses using gas chromatography or gas chromatography/mass spectrometry (GC/MS), the analytes must be volatile enough to partition between gas and liquid phase. Some analytes have strong enough intermolecular forces to hold them to each other or to a liquid solvent that they are insufficiently volatile to be suitable for GC/MS. They must be made volatile by the addition of a chemical group that reduces intermolecular forces; this process is called derivatization, and it is a routine practice. Derivatization may also make compounds easier to detect and serve other purposes.
In most but not in all states, CBD is legal. If cannabidiol (CBD) and D9-tetrahydrocannabinol (D9-THC) must be differentiated, then the choice of derivatization methods is critical to success. Using trifluoroacetic anhydride (TFAA) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) causes CBD to convert into a mixture of D8-tetrahydrocannabinyl trifluoroacetate and D9-tetrahydrocannabinyl trifluoroacetate of variable proportions. Treating D9-tetrahydrocannabinol with TFAA/HFIP produces the same mixture. One might call CBD a false positive for D9-THC using this method, but false positives usually refer to compounds that are distinct from the analyte in question. In this instance D9-THC and CBD are converted to a mixture of the same two compounds.
If chlorotrimethylsilane (TMSCl) is used as the derivatization reagent instead, D9-THC and CBD are converted into to easily differentiated compounds. The difference is that TMSCl does not promote the cyclization reaction of CBD that makes it strongly resemble THC, whereas trifluoroacetic anhydride does promote the cyclization reaction. The chemistry is shown below, using a drawing convention that does not indicate carbon or hydrogen atoms explicitly. Carbon atoms are understood to be at vertices and the ends of line segments. Each carbon atom holds the number of hydrogen atoms necessary to give it eight electrons. O is oxygen, and F is fluorine.
A journal article reported on this problem in 2012, yet an unknown number of laboratories still use the less discriminating method using TFAA. The issue was contested in at least one court case, and the testimony of an expert witness for the defense was decisive.
For further reading
Andrews R, Paterson S 2012 Journal of Analytical Toxicology 36:61-65. doi:10.1093/jat/bkr017.
Macherone A 2020 Cannabis Science and Technology September 3(7):42-48.